Synesthesia is a remarkable condition where stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. People with synesthesia experience a blending or crossing of their senses, such as sight and sound, leading to cross-modal sensations. In this condition, the stimulation of one sense, such as hearing a number or musical note, triggers an experience of another sense, like sensing a specific color, taste, or smell. People with synesthesia might experience hearing sounds when they see colors, a phenomenon sometimes called "colored hearing," or even tasting or smelling colors. For instance, hearing music might activate color-sensitive regions of the cortex and trigger a sensation of color, while seeing a number may evoke a taste or color sensation. These sensory shifts illustrate how individuals with synesthesia experience a blended perception of their environment.

Synesthesia can be considered an extreme version of the cross-modal responses that most people experience occasionally, such as associating high tones with lighter colors and low tones with darker colors. There are various types of synesthesia, each involving different sensory pairings.

Chromesthesia, for example, is when individuals see colors upon hearing sounds. A common example is hearing a violin and seeing the color purple or hearing a car horn and perceiving a flash of bright yellow.

Lexical-gustatory synesthesia involves certain sounds or spoken words triggering taste sensations. For example, hearing the word "piano" and tasting apples or the sound of a bell evoking the flavor of chocolate.

Mirror-touch synesthesia occurs when the observer sees someone else being touched and feels the same touch on their own body. For instance, if they see someone being tapped on the back, they might feel a tap on their own back.

Olfactory-visual synesthesia involves specific smells triggering visual experiences of colors or shapes. For example, the smell of fresh bread might cause someone to see swirling patterns of gold or the scent of roses might evoke the image of pink circles.

Lastly, Auditory-tactile synesthesia occurs when hearing certain sounds induces physical sensations on the skin. A loud siren might create a feeling of pressure on the chest, or a high-pitched whistle might produce a tingling sensation on the scalp.